I prefer doing the driver mod first.
This way you know how to drive most efficiently from the get go and can use the power to it's fullest advantage as well as keep the car under control and not smashed up like so many WRX's and other high-powered inexpensive cars end up.
From there, with any turbo car, your best bang for buck is the exhaust. Forced Induction(FI) cars need to breath much more than a NA car simply because the high volume of air that a FI car sucks and exhales.
As for suspension, a GOOD set of tires will be the best bang for the buck improvement to handling. It's been a while since I've bene buying performance rubber so things may have changed, but there were a few tires that stood out at autocross while still being road-legal. They were Hankook Ventus RS-2's and Falken Azenis used to reign supreme. Like I said though, I've been out of the autocross game for a few years, so I'm sure some other tires have come up in popularity.